1. Technical Field
This invention relates to ophthalmic implants and the manipulation of the intraocular pressure of the eye, and, more particularly, to a new and useful method and apparatus for lowering intraocular pressure by draining aqueous humor from the anterior chamber of the eye and by mechanically inhibiting wound healing at the surgical site.
2. Background
Glaucoma is a disorder characterized by increased intraocular pressure. It may cause impaired vision, ranging from slight sight loss to absolute blindness as a result of glaucomatous optic neuropathy. The causes of glaucoma are poorly understood; however, vasomotor and emotional instability, hyperopia, and especially heredity are among the predisposing factors. The increased intraocular pressure incident to glaucoma is related to an imbalance between production and outflow of aqueous humor, the watery fluid that fills the anterior and posterior chambers of the eye. Aqueous humor is secreted by the ciliary processes of the eye, and passes through the posterior chamber and the pupil into the anterior chamber where it is reabsorbed into the venous system at the iridocorneal angle by way of the sinus venosus, also called the canal of Schlemm. Obstruction of aqueous humor outflow appears to be mainly responsible for elevated intraocular pressures.
Glaucoma is a significant healthcare problem with immediate and long term ramifications, both physical and financial. Glaucoma is responsible for approximately ten percent of all blindness worldwide and is the leading cause of blindness in the United States, where over 300,000 new cases are reported each year. In the U.S., more than 95,000 glaucoma patients lose some degree of sight each year due to the disease, with greater than 5,500 experiencing total blindness. As treatment costs on a per year basis are estimated to be $1.5 billion, based upon more than two million annual office visits, the socio-economic impact of glaucoma is significant.
Along with persons predisposed to glaucoma by virtue of family history, individuals at higher risk of developing glaucoma are those 35 years of age or older or those with diabetes or positive glucose tolerance tests. The disease also strikes African-Americans in disproportionate numbers. They are four to five times more likely to develop glaucoma and are up to six times more apt to suffer a complete loss of sight.
The aim of current glaucoma treatment is the prevention of optic nerve damage and vision loss by lowering intraocular pressure. Common treatments for glaucoma include the systemic use of carbonic anhydrase inhibitors or topical applications of pilocarpine, timolol maleate, betaxolol HCl, levobunolol, metipranolol, epinephrine, dipivefrin, demecarium bromide, and echothiophate iodide. But, as is the case with most significant chemical therapies, the side effects of these medications may be severe while the treatment's efficacy is variable. When medication fails to control intraocular pressure, or visual fields show progressive defects, various glaucoma operations have been used to improve aqueous humor drainage. It has been reported that in 1993 over 1,152,000 operations for regulating intraocular pressure were performed in the U.S. alone.
While laser trabeculoplasty (alteration of the trabecular meshwork) has been utilized to a limited extent to improve aqueous drainage, glaucoma filtration surgery is the most widely practiced procedure when dealing with severe glaucoma cases. The fundamental principle of this surgery is to create an opening, or fistula, at the limbal region of the eye to facilitate the drainage of the aqueous humor, bypassing the pathological blockage in the anterior chamber angle. There are two basic approaches currently in use. In a full-thickness filtration procedure, a full-thickness sclerostomy is made, connecting the anterior chamber directly to the subconjunctival space. The main advantage of this procedure is the significantly lower intraocular pressures achieved postoperatively. However, because of its complications, this surgery is less frequently used than the second type of surgery, the trabeculectomy. In the trabeculectomy, a sclerostomy is performed under a scleral flap. This flap is then sutured back to its original bed in an attempt to minimize aqueous outflow runoff. The advantage of the trabeculectomy under the scleral flap is the tamponate effect provided by the resutured sclera causing a subsequent reduction of aqueous flow-through. Unfortunately, although this procedure provides short-term postoperative stability, final intraocular pressure levels are usually higher than those seen after full-thickness filtration, and the long term success rate is lower.
A problem with both these procedures, and glaucoma filtration surgery in general, is the body's natural healing process. Glaucoma filtration surgery differs from most surgical procedures in that inhibition of wound healing is desirable to achieve surgical success. When normal wound healing occurs, filtration rates decrease and intraocular pressures rise, making necessary the inhibition of the healing response. Surgical failures occur most frequently due to an overwhelming wound healing response and scarring of the filtering site. Histological studies of human and lab animal surgeries suggest that failure of glaucoma filtration surgery is associated with the presence of dense fibrovasular connective tissue around the surgical site. This prevents diffusion of the aqueous humor from the subconjunctival space.
To defeat the healing process, aqueous shunts have become an increasingly popular and effective means of lowering intraocular pressure. Among the various devices heretofore used, translimbal equatorial shunts have proven most effective. Examples of such devices include those disclosed in United States patents granted to Molteno (U.S. Pat. Nos. 4,457,757 and 4,750,901), Odrich (U.S. Pat. Nos. 5,041,081 and 5,127.901), and Baerveldt et al. (U.S. Pat. No. 5,178,604). Molteno's devices generally consist of ridged plates having drainage tubes for insertion into the anterior chamber of the eye. Odrich's patents disclose two ophthalmic implants for relieving pressure in the anterior chamber, both having one-way flow resisting valves, and residing under the conjunctiva. Baerveldt's apparatus comprises an elastomeric plate having a drainage tube tunneled through Tenon's capsule and the cornea and inserted into the anterior chamber.
The major disadvantage of current aqueous drainage devices is excessive aqueous drainage in the immediate postoperative period resulting in a flat anterior chamber and potential choroidal detachment. Profound hypotony, possibly leading to phthisis bulbi, is also a substantial risk. Excessive postoperative aqueous flow also causes expansion of the fibrous capsule beneath the rectus muscles of the eye. This mass effect stretches and tightens the muscles inducing heterotropia and motility restriction into the quadrant of the implant. A mass effect also may be exerted simply by the bulky presence of the device itself beneath the muscle causing scleral erosion, changes in eye curvature, or damage to adjacent vasculature and tissue. This is particularly true of rigid plastic or metal implants. Further problems with glaucoma filtration surgery include the conventional need for peripherial iridotomies, wherein a transverse division of some of the fibers of the iris is performed to create a communication between the anterior chamber and the posterior chamber. Other problems involve friction and wear imparted to the scleral flap by implanted devices, irritation of the iris endothelium caused by placement of implants into the anterior chamber, and aggravation produced by chronic forward and backward movement of the implants. In spite of these shortcomings, aqueous drainage devices have been successful in many cases, but the operative procedure remains challenging and significant complications are not unusual.